About one-third of 325 monoclonal antibodies with an apparent specificity for certain cancers are directed against carbohydrates. Of these, 7 are directed against the H antigen, 4 against the Le-b antigen, 3 against the B antigen, 55 against lacto-N-fucopentaose III, 2 against the sialylated Le-a antigen, and 26 against unidentified carbohydrate sequences in glycolipids and/or glycoproteins. The antibodies directed against the sialylated Le-a antigen may be useful diagnostically as they detect antigen in the serum of most patients with gastrointestinal and pancreatic cancer. Although the sialytated Le-A sequence of sugars occurs in the gangliosides of pancreatic and gastrointestinal cancers, the antigen detected in the sera of patients are mucins (high molecular weight carbohydrate-rich glycoproteins).